Blender 2.8, how to add a new shortcut
It looks to me that it still needs to be implemented as is literally missing from the options even if you want to modify the existing one. 

But for now you could do a quick addon, install it and use it as a shortcut.

if you use this script then the operator name is object.shading_workspace

don't know if there is a simpler way.

bl_info = {
    "name": "Shading Workspace",
    "author": "Your Name Here",
    "version": (1, 0),
    "blender": (2, 80, 0),
    "location": "",
    "description": "Shading Workspace",
    "warning": "",
    "wiki_url": "",
    "category": "",
}


import bpy


def main(context):
   for ob in context.scene.objects:
       print(ob)


class SimpleOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.shading_workspace"
    bl_label = "Shading Workspace"

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def execute(self, context):
        main(context)

        # this is the line you need
        bpy.context.window.workspace = bpy.data.workspaces["Shading"]

        return {'FINISHED'}


def register():
    bpy.utils.register_class(SimpleOperator)


def unregister():
    bpy.utils.unregister_class(SimpleOperator)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.object.shading_workspace()